Not sure whats up here. I run an AMD 1090T PC (6 core, 3.2GHz) which is a bit slower than the i7-2600K, so I'm likely to see more improvement with a GPU.

There's some overhead to offloading a bit of work to the GPU, or pretty much any loosely-coupled multiprocessing situation like this. I'm surprised it's necessarily slower than with the GPU acceleration disabled, but it's understandable you might not get as much of an improvement.

When I upgraded to Vegas 11 last fall, I bought both the nVidia GeForce GTX570 and the AMD/ATi Radeon HD6970... I was looking for acceleration per dollar, nothing more. These are the exact cards.

I actually found that the GTX570 underperformed vs. the AMD on every benchmark I tried... not what I expected from Sony's info. More to the point, the GTX570 didn't really help improve things much on my system at all during editing. Not that the HD6970 always did, either -- plain playback of a single video track still runs realtime at full quality, but the GPU isn't helping a great deal here.

The main advantage of the GPU on playback seems to be accelerating various plug-ins and effects, complex projects. The published Vegas "benchmark" project, for example, ran extremely well in Vegas 11 with the HD6970, maybe a little bit better with the GTX570.

At the time, I was doing a bunch of very complex animations, and the HD6970 helped quite a bit more here, too.

For both GPUs, there's overhead visible when using them. As GPU activity increases, CPU activity will decrease -- that's the CPU waiting on various work offloaded to the GPU to complete. In short, it makes sense, and it's not a bad thing if you have other stuff going on during a render.

The end result isn't day and night, but I'm pretty happy with the boost given by the HD6970 for $300... considerably less than it would cost to upgrade to a faster main PC. If I already had that faster main PC, I'm not sure I'd see enough of an improvement to justify the GPU. And it's highly dependent on what you're doing with it.

As for the slow-down, no explanation there... it shouldn't run slower. That sounds like a potential problem with that version of the nVidia driver. Not having used the nVidia stuff in awhile, I can't really comment... I'd check around the forums, here and on Sony's site. You can't be the first one to encounter this issues.